With the growth of the global economy, the demand for lower olefins such as ethylene and propylene has been increasing. Current production of ethylene and propylene still heavily depend on petroleum. For instance, in China, 60% of ethylene is from steam cracking of naphtha, and propylene is mainly from by-products of steam cracking and catalytic cracking (FCC) of naphtha. Due to the over-exploitation of petroleum resources, the reservoir and supply of petroleum diminish over time, Therefore, the route for preparing lower olefins via methanol or dimethyl ether from coal or natural gas as the starting material, i.e. the methanol-to-olefin (MTO) process or the methanol-to-propylene (MTP) process, is the most promising alternative to the naphtha route for preparation of lower olefins such as ethylene and propylene. At present, the main active component of a catalyst for MTO is the aluminum phosphate molecular sieve SAPO-34, and the main active component of a catalyst for MTP is the ZSM-5 molecular sieve. The two molecular sieves have different acidity and pore structures, and thus are used in methanol conversion processes for production of ethylene and propylene, respectively.
SAPO-34 molecular sieve is a molecular sieve based on silicon, phosphorus and aluminum, developed by Union Carbide (US) (U.S. Pat. No. 4,440,871), has a Chabazite (CHA)-based channel structure having both 8- and 4-membered rings which interconnect to form a cage, and has an effective pore size of 0.43 to 0.50 nm. The ZSM-5 zeolite molecular sieve is a high-Si zeolite having a 2D straight channel structure developed in 1970s by Mobil (US). It has an MFI-type topological structure, belongs to the orthorhombic system, and is formed by connected TO4 (T=Si, Al, Fe, etc.) tetrahedrons.
Although SAPO-34 and ZSM-5 show good catalytic performance in conversion of methanol into lower olefins, most catalysts for MTO or MTP currently used in industries are prepared by a “semi-synthetic” method in which a chemically synthesized molecular sieve is mixed with water, a matrix, a binder and the like and the mixture is stirred, beaten, dried and molded to obtain the catalysts. Catalysts prepared by such a “semi-synthetic” method have disadvantages such as non-uniform distribution of active components and blockage of channels by the binder.